Liquefaction of gases



June 1966 H. HASHEMl-TAFRESHI 3,257,813

LIQUEFACTION OF GASES Filed June 1, 1961 lnven B ommu fl, ylluiw /M Attorney United States Patent 3,257,813 LTQUEFACTION 0F GASES Hadi Hashemi-Tafreshi, London, England, assignor to Conch International Methane Limited, Nassau, Bahamas, a company of the Bahamas Filed June 1, 1961, Ser. No. 114,199 Claims priority, application Great Britain, Aug. 3, 1960, 26,847/60 15 Claims. (Cl. 6223) This invention relates to'the liquefaction of a gas containing minor proportions of a lower boiling gas. Thus, for example, it relates to the liquefaction of natural gas which consists mainly of methane with minor proportions of other gases, some of which, e.g. nitrogen, are lower boiling than methane.

While the invention is particularly applicable to the liquefaction of natural gas it is applicable to the liquefaction of any gas containing minor proportions-of a lower boiling gas, e.g. ethylene or ethane containing minor proportions of methane, oxygen containing minor proportions .of nitrogen, and nitrogen containing minor proportions of helium.

When gases containing minor proportions of lower boiling gases are liquefied in liquefaction cycles involving recycling of gaseous components there is a tendency for the proportion of the lower boiling gas in the system to increase and this eventually renders the liquefaction cycle unworkable. For example, on liquefying natural gas it is generally necessary to have a nitrgoen stripper in the cycle to remove nitrogen from the cycle and so avoid build up of nitrogen in the cycle.

I have now found a method of liquefying a gas containing minor proportions of a lower boiling gas which avoids such build up of the lower boiling component. This method not only does away with the need for special stripping arrangements but it is also extremely economical on the amount of power required to liquefy a given quantity of the gas.

According to the invention a method of producing a liquefied gas at apressure lower than the pressure at which it is liquefied from a compressed gas containing minor proportions .of a lower boiling gas comprises:

(a) Liquefying the compressed gas by indirect heat exchange with one or more refrigerants,

(b) Reducing the pressure on the liquefied compressed gas in two or more stages to produce, in the last decompression stage, liquefied gas at a desired pressure,

(c) Venting the gaseous phase produced in the last decompression stage from the system,

(d) Recompressing the gaseous phases produced in the other decompression stage or stages,

(e) Liquefying the rec-ornpressed gases from step (d) by indirect heat exchange with one or more refrigerants, and

f) Passing the liquid from (c) to the last decompression stage.

Preferably the last of the refrigerants used in step (e) is a stream of the liquefied gas taken from one of the decompression stages, which stream has been compressed and is, after use as refrigerant, returned to an earlier decompression stage.

A minor proportion of the re-compressed gases from step (d) may, after partial refrigeration, be fed into one, preferably the first, of the decompression stages.

Preferably, when there are three or more decompression stages, the gaseous phase from each of the decompression stages intermediate the first and last, is compressed to a pressure equal to the presure of the effiuent gas from the previous decompression stage and is fed into that effiuent gas.

- A part of the recompressed gas from step (d) may be "ice liquefied by heat exchange with the cold gas vented from the system and the liquid passed to the last decompression stage.

The refrigerants used may be of any type suitable to the temperature requirements and any suitable type of refrigerating systems may be used. Generally a compression refrigeration system will be employed using a material such as ammonia, propane, ethylene, ethane or methane. However, an absorption system may be useful in some instances.

The invention will now be described with particular reference to the liquefaction of natural gas and with reference to the accompanying drawing which is a flow sheet of the liquefaction of natural gas by a method according to the invention.

Natural gas which has been dried and which consists mainly of methane but which contains minor proportions of other gases including nitrogen and carbon dioxide, enters the liquefaction system by pipe 1 under a pressure of 1500 p.s.i.a. and at ambient temperature or above. It passes through heat exchanger 2 in which it is cooled to 20 F. by indirect heat exchange with liquid ammonia passing through pipe 3 under a pressure of 250 p.s.i.a. and at an inlet temperature of 25 F. The rest of the ammonia refrigeration cycle, of which pipe 3 is a part, is not shown since it forms no part of the invention.

The natural gas leaving heat exchanger 2 passes to heat exchanger 4 in which it is cooled to 200 F. by indirect heat exchange with liquid ethylene, at 320 p.s.i.a. and at an inlet temperature of 205 F., passing through pipe 5 which is part of an. ethylene refrigeration cycle, the rest of which is not shown on the drawing.

The natural gas leaves heat exchanger 4 in a liquefied condition and passes through expansion valve 6 into expansion chamber 7. The degree of expansion in this chamber is sufficient to reduce the temperature to 22S F. Liquid from expansion chamber 7 passes through expansion valve 8 to the second expansion chamber 9, the drop in pressure being sufficient to reduce the temperature in expansion chamber 9 to 248 F.

A part of the iliquid in expansion chamber 9, flows through expansion valve 16 and pipe 11 to the final expansion chamber 12. The reduction in pressure in expansion chamber 12 is sufiicient to cool the liquid to 269 F. and the pressure to substantially atmospheric. The liquid in expensio-n chamber 12 if fed via pump 13 to storage tank 14. The gas phase from expansion chamber 12 is vented from the system via pipe 15 which passes through heat exchanger 16 to be described in more detail hereafter. Similarly, the boiled off gases from the storage tank 14 are vented from the system via pump 35 and pipes 17 and 15. The vented gases, which are nitrogen enriched natural gas,'may be used as fuel gas for producing the power needed in the liquefaction system.

Dependent on the C0 content of the original natural gas solid CO may be precipitated in particulate form in any of the expansion chambers 7, 9 and 12 and may be removed therefrom by any suitable means. As shown in the drawing, solid CO may be removed from expansion chamber 12 via pipe 18. The temperature to which the natural gas is reduced in heat exchanger 4 should be adjusted so that the CO in the gas remains in solution in the liquid.

The gases produced in expansion chamber 9 leave that chamber via pipe 19 and are compressed in compressor 24 preferably a centrifugal compressor, to apressure equal to that of the gases leaving expansion chamber 7. The gases leave expansion chamber 7 via pipe 21 and are mixed with the compressed gases from compressor 20 and the mixture fed to compressor 22, preferably a centrifugal compressor, in which the gases are compressed to 470 p.s.i.a. These compressed gases are passed through compressor 23, preferably a two stage reciprocating compressor, and a water cooler 24, and are then fed at 1500 p.s.i.a. and about 100 F. via pipe 25 through heat exchanger 2 in which they are cooled to F. and then through heat exchanger 4 in which they are cooled further to 200 F., in both cases by indirect heat exchange with the refrigerants already described.

After leaving heat exchanger 4 the now liquefied gases are fed through pipe 26 and heat exchanger 27. In heat exchanger 27 the liquefied recycled gases are further cooled to 243 F. by indirect heat exchange with liquefied natural gas which has been withdrawn via line 28 from expansion chamber 9, compressed to 100 p.s.i.a. in compressor 29 and passed through heat exchanger 27 in pipe 30. After acting as a refrigerant this liquefied natural gas passes through expansion valve 31 into expansion chamber 7.

The cold liquefied recycled gas in pipe 26, after passing through heat exchanger 27 is fed via expansion valve 32 and pipe 11 to expansion chamber 12.

A part of the compressed recycled gases in pipe may be fed via pipe 33 through heat exchanger 16 in which it is cooled by indirect heat exchange with the efiluent gases from expansion chamber 12 to a temperature of 255" F. and then fed via expansion valve '34 back to expansion chamber 12.

In the process of liquefaction described above, with reference to the accompanying drawing the nitrogen in the natural gas is partially separated from it in the last decompression stage in expansion chamber 12. Some nitrogen is, of course, left in the liquefied natural gas going to storage but this is no demerit since it reduces the boil off losses in storage and in subsequent transport. It will be clear that since the recycled gases pass only to the last decompression stage there can be no accumulation of nitrogen in the cycle.

In fact the process will tolerate the return of a small proportion of the recycled gases to deeompresison stages prior to the last one; In fact it may be desirable to do this in order to adjust the heat balance around heat exchanger 27, and in the accompanying drawing some of the liquefied recycled gases may be fed via expansion valve to expansion chamber 7.

While in the foregoing description carbon dioxide in the natural gas was not removed prior to it entering the liquefaction cycle, it will be clear that the natural gas going to the cycle can be submitted to any desirable pretreatment, f-or example, to remove acid gases such as carbon dioxide and hydrogen sulphide, or to remove heavier hydrocarbons. It is also possible to remove heavier hydrocarbons between heat exchangers 2 and 4 where they may well preferentially condense out, depending, of course, on their concentration and precise nature. In fact, hydrocarbons heavier than methane which condense out at any stage in the liquefaction cycle can be removed prior to the liquefaction of the methane.

It will be appreciated that in the foregoing description with the reference to the accompanying drawing, the pressures and temperatures given are merely illustrative and are capable of quite wide variation even when liquefying natural gas. Thus, in the example, the natural gas was fed into the liquefaction cycle at 1500 p.s.i.a. but in fact any suitable pressure can be employed although generally this will lie between 700 and 1600 -p s.i.a. Also, in the example, the product was liquefied natural gas at atmospheric pressure. The product liquefied gas may be required at some higher pressure in some cases and in that case the decompression stages would be such as to take the pressure down to that desired for storage and transport.

I claim:

1. A method of producing a liquefied gas at a pressure lower than the pressure at which it is liquefied from a compressed gas containing minor proportions of a lower boiling gas which comprises:

(a) liquefying the compressed gas by indirect heat exchange with at least one refrigerant,

(b) reducing the pressure on the liquefied compressed gas in a plurality of stages to produce, in the last decompression stage, liquefied gas at a desired pressure,

(c) venting the gaseous phase produced in the last decompression stage from the system,

((1) recompressing the gaseous phases produced in the other decompression stages,

(e) liquefying the recompressed gases from step (d) by indirect heat exchange with at least one rcfrigerant, and

(f) passing the liquid from (e) to the last decompression stage.

2. A method as claimed in claim 1, in which there are at least three decompression stages, the gaseous phase from each of the decompression stages intermediate the first and last, is compressed to a pressure equal to the pressure of the efiluent gas from the previous decompression stage and is fed into that efiiuent gas.

3. A method as claimed in claim 1, in which part of the recompressed gas from step (d) is liquefied by heat exchange with the cold gas vented from the system and the liquid is passed to the last decompression stage.

4. A method as claimed in claim 1, in which the gas is natural gas consisting mainly of methane containing minor proportions of other gases, including nitrogen and carbon dioxide.

5. A method as claimed in claim 4, in which the natural gas is at the pressure of between 700 and 1600 p.s.i.a. when fed to step (a) and the product is liquefied natural gas at substantially atmospheric pressure.

6. A method as claimed in claim 4 wherein solid CO precipitated in particulate form in any of the expansion chambers, is removed from the system.

7. A method of producing a liquefied gas at a pressure lower than the pressure at which it is liquefied from a compressed gas containing minor proportions of a lower boiling gas which comprises:

(a) liquefying the compressed gas by indirect heat exchange with at least one refrigerant,

(b) reducing the pressure on the liquefied compressed gas in a plurality of stages to produce, in the last decompression stage, liquefied gas at a desired pressure,

(c) venting the gaseous phase produced in the last decompression stage from the system,

(d) recompressing the gaseous phases produced in the other decompression stages,

(e) feeding a minor proportion of the recompressed gases from step (d) after partial refrigeration, into one of the decompression stages,

(f) liquefying a major proportion of the recompressed gases from step (d) by indirect heat exchange with a plurality of refrigerants in series, the last of which is a stream of the liquefied gas taken from one of the decompression stages, which stream has been compressed and which is, after use as a refrigerant, returned to an earlier decompression stage, and

(g) passing the liquid from (f) to the last decompression stage.

8. A method as claimed in claim 7 in which the minor proportion of recompressed gases is fed into the first of the decompression stages.

9. A method as claimed in claim 8 in which there are at least three decompression stages, the gaseous phase from each of the decompression stages intermediate the first and last, is compressed to a pressure equal to the pressure of the efiiuent gas from the previous dccompression stage and is fed into that efiiuent gas.

10. A method as claimed in claim 8 in which a part of the recompressed gas from step (d) is liquefied by heat exchange with the cold gas vented from the system and the liquid is passed to the last decompression stage.

11. A method as claimed in claim 8 in which the gas is natural gas consisting mainly of methane and containing a minor proportion of nitrogen and is at a pressure of 700 to 1600 p.s.i.a. when fed to step (a) and the product is liquefied natural gas at substantially atmospheric pressure.

12. A method of producing liquefied natural gas at a pressure lower than the pressure at which it is liquefied from compressed natural gas containing minor proportions of nitrogen which comprises:

(a) liquefying the compressed gas by indirect heat eX- change with at least one refrigerant,

(b) reducing the pressure on the liquefied compressed gas in a plurality of stages to produce, in the last decompression stage, liquefied gas at a desired pressure,

(c) venting the gaseous phase produced in the last decompression stage from the system,

(d) recompressing the gaseous phases produced in the other decompression stages,

(e) feeding a minor proportion of the recompressed gases from step (d) after partial refrigeration, into one of the decompression stages,

(f) liquefying a major proportion of the recompressed gases from step (d) by indirect heat exchange with a plurality of refrigerants in series, the last of which is a stream of the liquefied gas taken from one of the decompression stages, which stream has been compressed and which is, after use as a refrigerant, returned to an earlier decompression stage, and

(g) passing the liquid from (f) to the last decompression stage.

13. A method of producing a liquefied gas at a pressure lower than the pressure at which it is liquefied from a compressed gas containing minor proportions of a lower boiling gas which comprises:

(a) liquefying the compressed gas by indirect heat exchange with at least one refrigerant,

(b) reducing the pressure on the liquefied compressed gas in a plurality of stages to produce, in the last decompression stage, liquefied gas at a desired pressure,

(c) venting the gaseous phase produced in the last decompression stage from the system,

(d) recompressing the gaseous phases produced in the other decompression stages,

(6) liquefying the recompressed gases from step (d) by indirect heat exchange with at least one refrigerant,

(f) passing the liquid from (e) to the last decompres sion stage, (g) compressing a stream of liquefied gas taken from one of the decompression stages after the first one,

(h) using a plurality of refrigerants in series in step (e), the last of which refrigerants is the compressed stream of liquefied gas of step (g),

(i) thereafter returning said compressed stream to an earlier decompression stage.

14. A method of producing a liquefied gas at a pressure lower than the pressure at which it is liquefied from a compressed gas containing minor proportions of a lower boiling gas which comprises:

(a) liquefying the compressed gas by indirect heat exchange with at least one refrigerant,

(b) reducing the pressure on the liquefied compressed gas in a plurality of stages to produce, in the last decompression stage, liquefied gas at a desired pressure,

(0) venting the gaseous phase produced in the last decompression stage from the system,

(d) recompressing the gaseous phases produced in the other decompression stages,

(e) liquefying the recompressed gases from step (d) by indirect heat exchange with at least one refrigerant,

(f) passing the liquid from (e) to the last decompression stage,

(g) partially refrigerating a minor proportion of the recompressed gases from step (d), and

(h) feeding said minor proportion to one of the decompression stages.

15. A method as claimed in claim 14, in which said minor proportion is fed into the first of the decompression stages.

References Cited by the Examiner UNITED STATES PATENTS 1,574,119 2/ 1926 Seligmann 62-9 2,145,130 1/1939 Reich.

2,258,749 10/1941 Eaton 62--28 X 2,500,118 3/1950 Cooper.

2,500,129 3/ 1950 Laverty 62-27 2,712,738 7/1955 Wucherer 6213 X 2,811,843 11/1957 Morrison 629 2,812,646 11/1957 Twomey *62-40 X 2,896,414 7/1959 Tung 62--23 X 2,900,797 8/ 1959 Kurata 62--12 2,901,326 8/1959 Kurata 6212 2,973,834 3/1961 Cicalese.

2,996,891 8/1961 Tung.

3,020,723 2/ 1962 De Lury.

NORMAN YUDKOFF, Primary Examiner.

RICHARD A. OLEARY, Examiner.

R. C. STEINMETZ, Assistant Examiner. 

1. A METHOD OF PRODUCING A LIQUEFIED GAS AT A PRESSURE LOWER THAN THE PRESSURE AT WHICH IT IS LIQUEFIED FROM A COMPRESSED GAS CONTAINING MINOR PROPORTIONS OF A LOWER BOILING GAS WHICH COMPRISES: (A) LIQUEFYING THE COMPRESSED GAS BY INDIRECT HEAT EXCHANGE WITH AT LEAST ONE REFRIGERANT, (B) REDUCING THE PRESSURE ON THE LIQUEFIED COMPRESSED GAS IN A PLURALITY OF STAGES TO PRODUCE, IN THE LAST DECOMPRESSION STAGE, LIQUEFIED GAS AT A DESIRED PRESSURE, (C) VENTING THE GASEOUS PHASE PRODUCED IN THE LAST DECOMPRESSION STAGE FROM THE-SYSTEM, (D) RECOMPRESSING THE GASEOUS PHASES PRODUCED IN THE OTHER DECOMPRESSION STAGES, (E) LIQUEFYING THE RECOMPRESSED GASES FROM STEP (D) BY INDIRECT HEAT EXCHANGE WITH AT LEAST ONE REFRIGERANT, AND (F) PASSING THE LIQUID FROM (E) TO THE LAST DECOMPRESSION STAGE. 